The present invention relates to a method of purifying alkoxysilanes of chlorine-containing impurities. Alkoxysilanes, especially with sterically demanding groups, are finding increasing interest in the fields of application of building protection and cable masses. The purity requirements as regards the acid, base and particularly the chloride content are extremely high, especially for organosilanes used in the cable industry.
Alkoxysilanes with the general formula R.sub.n Si(OR').sub.4-n are obtained by liquid-liquid, gas-liquid or gas-gas phase reaction of the corresponding chlorosilanes with alcohol. The raw products obtained after the esterification still contain a residual acid content which can be demonstrated analytically as HCl and which can be traced to certain components of the so-called "monochloro derivative" as a consequence of the incompletely performed conversion during the alcoholysis. Such a method is described e.g. in European Patent 421,644.
Chung and Hayes (Journal of Organometallic Chemistry (1984), 265: 135-139) describe a method for the removal of chlorine-containing impurities in alkoxysilanes using lithium aluminum hydride or metallic sodium. The hydrolyzable chlorine content was lowered to below 100 mg/kg only in the presence of sodium. It is known that alkali metals fall into the group of interfering substances and that these methods necessitate an extensive safety analysis when used on a large industrial scale and can cause safety problems
In European Patent 0,223,210 there is described a purification method for organosilanes of the general formula R.sub.n Si(OR').sub.4-n with the terminal groups R= C.sub.1--8. The method uses aoidic clays or iron chloride in which very low concentrations of chlorine-containing impurities are likewise achieved. This method operates at normal pressure and preferably at the boiling temperature of the alkoxysilane in question. However, a thermally induced condensation occurs thereby, especially in the presence of strong bases, depending on the structure of the organosilane. The alcohol which is freed in the course of the reaction lowers the treatment temperature. This generally results in prolonging the dwell time and, as a consequence of the silane formation, lower yield.